Outliers and
Extremes

When creating box plots (either 2D Box Plots
or 2D Scatterplots with Box
Plots), you can select to display none (select Off),
only Outliers, only Extreme
values, or both Outliers &
Extremes in the box plot. These options are available in the Outliers group box on the respective
Advanced tabs.

Outliers
and Extremes. Values that are "far" from the middle
of the distribution are referred to as outliers and extreme values if
they meet the conditions specified below.

Outliers. For Graphs
menu box plots, a data point is deemed to be an outlier if the following
conditions hold:

data point value > UBV
+ o.c.*(UBV - LBV)

or

data point value < LBV
- o.c.*(UBV - LBV)

where:

UBV

is the upper value of the box in the box plot (e.g., the mean
+ std.err. or the 75th percentile).

LBV

is the lower value of the box in the box plot (e.g., the mean
- std.err. or the 25th percentile).

Extremes. For Graphs
menu box plots, a data point is deemed to be an extreme value if the following
conditions hold:

data point value > UBV
+ 2*o.c.*(UBV - LBV)

or

data point value < LBV
- 2*o.c.*(UBV - LBV)

where:

UBV

is the upper value of the box in the box plot (e.g., the mean
+ std.err. or the 75th percentile).

LBV

is the lower value of the box in the box plot (e.g., the mean
- std.err. or the 25th percentile).

o.c.

is the outlier coefficient specified in the Outliers
option on the 2D
Box Plots - Advanced tab (by default, the outlier
coefficient is 1.5, thus, the extreme values are those which are outside
the 3 box length range from the upper and lower value of the box; for
example, see 2D
Box Plots)

The following diagram illustrates the ranges of outliers and extremes
in the "classic" box and whisker plot where the upper box value
(UBV) is the 75th percentile, the lower box value (LBV) is the 25th percentile,
and the outlier coefficient is equal to 1.5.